There are cases where an etching structure (hole or trench) formed by etching a silicon substrate is formed into a tapering shape with a wide top opening width and a narrow bottom width by forming the side wall thereof to be not vertical but inclined as shown in FIG. 3(b).
As an etching method of forming such a tapering etching structure in a silicon substrate, conventionally, a method disclosed in the Japanese Unexamined Patent Application Publication No. H2-89310 has been known, for example. This etching method is a method in which a gas mixture of hydrogen bromide and oxygen is used as an etching gas, and plasma is generated from this gas mixture to thereby etch a silicon substrate.
In this etching method, because an oxide film as an etch-resistant layer is formed on the silicon substrate by generating plasma from the oxygen gas and the silicon substrate is etched by generating plasma from the hydrogen bromide gas, the etching proceeds while the side wall of the etching structure is protected by the oxide film, and thereby the etching structure formed by the etching is formed into a tapering shape.